Recent advances in medical technology have resulted in the development of a variety of medical devices for permanent or temporary implantation in the human body. Effective positioning of such devices can prove to be a very difficult task, and maintaining an implanted device in a desired position for an extended period of time is often more difficult. This is particularly true if the implanted device is to remain only temporarily and is designed to facilitate subsequent removal.
A number of medical implant devices are designed to collapse for insertion within a catheter or other delivery unit and to expand to a predetermined shape when ejected after delivery. Many of these self expanding devices rely primarily upon the contact between the device and the wall of a body vessel or passageway to maintain the device in position after the delivery unit is removed. Unfortunately, changes in the dimensions of the body vessel or passageway or variations in the flow of blood or other fluids there through can cause the medical implant to migrate and change position.
It is extremely important that a medical implant device be properly positioned and oriented, and that this position and orientation be maintained. Otherwise, effective performance of such therapeutic devices will not be achieved. It is often very difficult to move such a device into position with the desired orientation, and once this is achieved, it is critical that no further motion occur.
In an attempt to prevent migration of a medical implant device, rigid hooks are often formed on the device to engage the wall of a body vessel or passageway as the implant device expands into contact with the wall. After a few weeks, the endothelium layer grows over rigid hooks which will not easily bend under the influence of withdrawal pressure, and the medical implant device will be locked in place by the embedded hooks. This may be acceptable for a permanent implant, but rigid hooks are not a viable option if the medical implant device is to be removed after several weeks or months.
To facilitate removal of a previously implanted medical device by withdrawal of the anchoring hooks from an enveloping endothelium layer without risking substantial damage to the wall of a body vessel or passageway, the hooks have been formed to straighten when subjected to a withdrawal force greater than a maximum migration force. U.S. Pat. Nos. 6,007,558 and 6,258,026 to Ravenscroft, et al show hooks which are formed to bend and straighten in response to a withdrawal force, while U.S. Pat. No. 4,425,908 to Simon, U.S. Pat. No. 4,817,600 to Herms, et al, U.S. Pat. No. 5,108,418 to Lefebvre, U.S. Pat. No. 5,133,733 to Rasmussen, et al, U.S. Pat. No. 5,242,462 to El-Nounou, et al, U.S. Pat. No. 5,370,657 to Irie, U.S. Pat. No. 5,601,595 to Smith, U.S. Pat. No. 5,800,457 to Gelbfish, and U.S. Pat. No. 5,853,420 to Chevillon, et al all disclose expandable medical implant devices; many with anchoring hooks.
Anchoring hooks, although effective in many instances, are subject to a number of disadvantages which can make it difficult to properly position and maintain the position of a medical implant device. In prior devices, the anchoring hooks are engaged due to the expansion of the device into contact with the wall of a body vessel or passageway, and if the device moves from a desired position during expansion and contact with the wall occurs, the device cannot be easily repositioned. The anchoring function of the hooks is not separable from the expansion of the device.
In cases where the operation of the hooks is tied to the expansion of a medical implant device, there can be instances where one or more of the hooks fails to properly engage the wall of a body vessel or passageway causing the device to become off center. Sometimes movement of the device longitudinally will engage the errant hooks, but this movement can also alter the position of the device.
Also, the configuration of a hook which curves in a single direction from a shaft to a pointed end can prove to be a disadvantage. When hooks are used to anchor a medical implant device within a blood vessel, it is important that the hook be oriented to curve in the direction of normal blood flow through the vessel as it engages the vessel wall. Thus when engaged, the hook will extend from the shaft toward the point substantially in the direction of the longitudinal axis of the blood vessel, and will effectively resist migration of the medical implant device in response to pressure thereon from blood flow in the normal direction through the blood vessel. However, there are conditions which can result in a backflow of blood in a blood vessel, and pressure on the device and the anchoring hooks resulting from such backflow can cause the hooks to back out and disengage from the vessel, thus changing the orientation of the device within the blood vessel and causing deleterious changes in the performance of the implant.
Finally, even if the hooks of an implant device are properly engaged with a vessel wall, there are conditions which result in the subsequent outward expansion of the vessel wall to an extent where the hooks tend to become disengaged.